feat: Improved representation for metadata in `hugr-model` #3254

Job	Run time
Check for changes in Python files	5s
Check serialization schema	7s
Build HUGR binary	49s
check python 3.10	13s
check python 3.13	22s
test python 3.10	27s
test python 3.13	32s
Required checks 🐍	1s
	2m 36s

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